Figure 8: Proposed mechanism of NF-κB activation via coactivation receptors on NK cells.

NF-κB activation in NK cells required the coordinated engagement of coactivation receptors, such as 2B4 and NKG2D or 2B4 and DNAM-1. This combination was required to provide complementary and independent signals leading to Vav1-dependent synergistic signalling involving PLC-γ2 and Erk. Further, signals from synergizing receptors converged on NF-κB p65 subunit through selective phosphorylation of p65 serine residues, particularly at serine 276 via Vav1-Erk and at serine 536 via PI3K-Akt pathway, which was crucial to optimal activation of NF-κB. The requisite PI3K-Akt signal was primarily mediated by the engagement of NKG2D or DNAM-1, which recognizes ligands induced by cellular stress. Vav1 controlled downstream p65 phosphorylation and NF-κB activation, suggesting that distinct signalling checkpoints at the level of Vav1 and p65 regulate NF-κB activation. In support, Vav1-dependent synergistic signalling was required for the phosphorylation of p65 at serine 536 by Akt pathway, which was evident in SAP-deficient XLP1 NK cells following coactivation, which exhibited impaired p65 phosphorylation, nuclear translocation, NF-κB activation and downstream NF-κB-dependent functions.